It is known that the extratropical (i.e. mid- and high-latitude) tropopause in the southern hemisphere is higher by about 1km in winter than in summer, whereas it stays at almost constant level irrespective of the season in the northern hemisphere (Appenzeller et al 1996). The tropopause is defined by a particular value of potential vorticity : PV=2.0PVU. Since the Brewer-Dobson circulation in the stratosphere is stronger in the northern hemispheric winter than in the southern hemispheric winter, one may relate the different seasonal change in the extratropical tropopause height to the difference of the Brewer-Dobson circulation between the northern and the southern hemispheres. To examine this, we consider how much the hemispheric difference of stratospheric wave forcing can explain the difference in the extratropical tropopause height.
First, we estimate the effect of the stratospheric circulation on the tropopause height by a simple model, in which the stratosphere radiation is given by the Newtonian cooling where temperature is relaxed to a prescribed temperature profile by a radiative damping time tau, and the tropospheric temperature is assumed to be unchanged. The induced change in the tropopause height by the residual circulation is estimated as dH = G_s/(G_t - G_s) * (tau * w), where G_s and G_t are the lapse rates in the stratosphere and troposphere, respectively, and w is the vertical velocity in the stratosphere. We choose G_t=6.5K/km, G_s=2.0K/km, tau = 40day, and motivated by observational estimation of interhemispheric difference, w=0.6mm/s. The resulting value of dH is 921m, which is comparable to the observed difference of the winter tropopause height between the northern and the southern hemispheres.
Next, we calculate a seasonal cycle of the general circulation by using an aqua planet model ( a general circulation model ) which includes solar absorption by ozone and hydrological cycle without topography. Sea surface temperature and solar radiation have a prescribed seasonal cycle. A prescribed seasonally variable momentum source is imposed in the stratosphere, corresponding roughly to the observed wave forcing. The effect of the momentum forcing on the tropopause height is investigated. We have made a comparison of the change in the heights by various definitions of the tropopause such as PV=2.0PVU and the lapse rate=2K/km. The results show that the change in the extratropical tropopause height is comparable to the estimated value for each definition of the tropopause. We also calculated the effective diffusivity proposed by Nakamura (1996) to identify the tropopause by using PV on the isentropic surfaces as a tracer field. For the two cases with and without the stratospheric forcing, PV is close to 2.0PVU at the height where the effective diffusivity becomes minimum. So this definition of the tropopause height also decreases with the stronger forcing in the stratosphere.
Since the stratospheric forcing has a small effect on the tropospheric circulation, the position of the subtropical jet does not change too much as a result of the forcing. Furthermore, there is no appreciable change in the tropical tropopause height, a result somewhat different from that of Thuburn and Craig (1998).
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12th Conference on Atmospheric and Oceanic Fluid Dynamics